Modular FACTS devices with external fault current protection

ABSTRACT

Flexible AC transmission system (FACTS) enabling distributed controls is a requirement for power transmission and distribution, to improve line balancing and distribution efficiency. These FACTS devices are electronic circuits that vary in the type of services they provide. All FACTS devices have internal circuitry to handle fault currents. Most of these circuits are unique in design for each manufacturer, which make these FACTS devices non-modular, non-interchangeable, expensive and heavy. One of the most versatile FACTS device is the static synchronous series compensator (SSSC), which is used to inject impedance into the transmission lines to change the power flow characteristics. The addition of integrated fault current handling circuitry makes the SSSC and similar FACTS devices unwieldy, heavy, and not a viable solution for distributed control. What is disclosed are modifications to FACTS devices that move the fault current protection external to the FACTS device and make them modular and re-usable.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/694,605 filed Sep. 1, 2017, which claims the benefit of U.S.Provisional Patent Application No. 62/527,873 filed Jun. 30, 2017, thedisclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to systems and methods for flexible ACtransmission systems (FACTS) and specifically to use of distributedpower transmission and distribution control by static synchronous seriescompensators and other FACTs devices.

2. Prior Art

FACTS based distributed control of transmission lines and connecteddistributed generation capabilities and loads have become very criticalfor improving the efficiency of the power grid. In flexible ACtransmission systems (FACTS), power flow control devices vary in thetype of services they can provide. Devices operate in either series orshunt modes, and are highly complex and sophisticated pieces ofmachinery that require long planning cycles and preparation beforeinstallation.

Many of the FACTS devices use high voltage semiconductor-based powerelectronic converters to control the required parameters, such as linecurrent, bus voltage, and more. Although converter-based FACTS devicesprovide more granular and faster control than electro-mechanical devicessuch as Phase Shifting Transformers, the former have significantlimitations in fault-handling capability. The cost and complexity of thefault-handling strategy and circuit design in a FACTS device is one ofthe significant limitations and is considered one of the main deterrentsfor large-scale adoption of the FACTS technology.

Furthermore, most FACTS devices today are custom-built for specificapplications, thus, no plug-and-play solution exists today. The lack ofa solution is due to the unique design of the fault handling capabilitydesigned and implemented by individual suppliers of the FACTS systemsand modules.

During a typical fault on the power grid very high currents appear onthe transmission lines of the grid. These fault conditions can be shortlived, such as those due to a lightning strike or they can be extendedsuch as those due to ground shorts. Since the electronic components andthyristor used in todays' FACTS devices are prone to failure when suchcurrents are impressed on them, these conditions must be handled by thefault protection circuitry. The high, short-duration faults aregenerally diverted away from sensitive semi-conductor switches (likeIGBTs) using fast acting, more robust switches such as SCRs,electro-mechanical contactors, etc. In additions the circuits may alsouse metal oxide varistors (MOVs) to limit voltage rise. MOVs have aresistance value that reduces with the voltage applied across it.

FIG. 1 shows a prior art implementation 100 of a thyristor controlledSeries compensator (TCSC) or a synchronous static series compensator(SSSC) 104 that includes the fault current protection as part of thepower grid system 100. The power system comprises: the generator 101,the transformer 102, for stepping up the voltage for transmission overthe transmission line 105. The circuit breaker (CB) 103 is used toisolate the generator 101 from the transmission line 105 and any FACTSdevices like TCSC or SSSC 104 in case of ground short 108. A secondbreaker 106 is used to isolate the power grid from the load 107. Duringregular operation, the TCSC or the SSSC 104 provide the capability forthe line to be efficiently used for transfer of power.

FIG. 2 is a prior art example of the series capacitive compensation forthe inductance of the power lines. As can be seen the protection circuitassociated with the capacitor 202 in series with the power line 201comprise the MOV bank 203 and a triggered gap 205, which may be a vacuumbottle, in series with an inductance 204 used to limit the currentthrough the vacuum bottle or in the case of longer time periods thebypass CB 206.

FIG. 3 is a prior art example of a single TCSC unit 307 with theassociated fault current protection circuits. The TCSC 307 with there-closer switch 306, and in combination with the inductor ‘L’ 305 inparallel with the capacitor ‘C’ 304 is able to inject both capacitive orinductive impedances on the power line 201 based on the firing of thethyristors, the control being provided by the firing angle and duration.The protection circuitry includes the MOV 203 stack, the triggeredair/vacuum gap 205, and the bypass breaker 206. The triggered air gap205 and the bypass breaker have the damping circuit 204 to reduceoscillations and provide a current limit. In addition to the faultcurrent protection the FIG. 3 also shows the circuit breakers 303 A and303 B which allow the TCSC module to be disconnected from the line 201and a re-closer breaker 302 for reconnecting the TCSC when a fault isrepaired.

These prior art FACTS based power flow control modules show the controlcircuits with the fault protection associated with it. The faultprotection makes the control units large and unwieldy. It is hence onlyefficient to have the power flow control modules in substations and notusable effectively in distributed control applications.

It will be ideal if the fault handling capability can be removed frominside the FACTS systems and modules to an external protection scheme.The individual FACTS modules and systems then become modular and capableof plug and play. In addition the modular FACTS devices are lighter andsmaller and can thus be useable in distributed applications on the grid.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are made to point out and distinguish the invention fromthe prior art. The objects, features and advantages of the invention aredetailed in the description taken together with the drawings.

FIG. 1 is a prior art system block diagram 100 of thyristor controlledseries compensator (TCSC) with breaker protections as part of the powergrid system.

FIG. 2 is a prior art block diagram 200 of a series capacitor bankincluding the fault current protection components.

FIG. 3 is an exemplary prior art block diagram 300 showing the internalcomponents including fault protection components of the TCSC of FIG. 1.(Prior Art)

FIG. 4 is an exemplary block diagram 400 of TCSC without fault currentprotection and its block representation 401 as per an embodiment of thecurrent invention.

FIG. 5 is an exemplary block diagram 500 of an SSSC without faultcurrent protection and its block representation 501 as per an embodimentof the current invention.

FIG. 6 is an exemplary block diagram 600 of an implementation of thefault current protection module and its single block representation 601as per an embodiment of the present invention.

FIG. 7 is an exemplary power grid system block diagram 700 with TCSC 401or SSSC 501 with external Fault current protection 601 and bypass andground isolation protections.

FIG. 8 is an exemplary diagram showing multiple SSSCs 501 s deployed inseries being protected by an external fault current protection module601, the SSSCs further being enabled for isolation and ground connectionusing breakers.

FIG. 9 is an exemplary deployment of two groups of TCSCs in seriesparallel configuration with external fault current protection.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The primary change to the FACTS devices is moving the unit-level faultprotection module external to the FACTS device. This provides:

Substantial reduction in volume and weight of the FACTS devices allowingthem to be used in (1) distributed applications; (2) applications wherea plurality of FACTS devices need to be configured and used as a group.In that regard, the reduction in volume allows heat generated within theFACTS devices to more readily pass out.

The system reliability is improved due to reduction in the number ofmodules/components used, that result in reducing the number of failurepoints or nodes within the implemented modules and sub-systems.

The removal of custom designed fault protection modules enablesstandardization of the FACTS modules for use in distributed applicationsrequiring lower cost.

Flexible AC transmission system (FACTS) enabling distributed controls isa requirement for power transmission and distribution, to improve linebalancing and distribution efficiency. These FACTS devices areelectronic circuits that vary in the type of services they provide. AllFACTS devices have internal circuitry to handle fault currents. Most ofthese circuits are unique in design for each manufacturer, which makethese FACTS devices non-modular, non-interchangeable, expensive andheavy. One of the most versatile FACTS device, the static synchronousseries compensator (SSSC) is used to inject impedance into thetransmission lines to change the power flow characteristics. Theaddition of integrated fault current handling circuitry makes the SSSCsand similar FACTS devices unwieldy, heavier and not viable as a solutionfor distributed control. What is disclosed are modifications to FACTSdevices that move the fault current protection external to the FACTSdevice and make them modular and re-usable.

FIG. 4 shows a TCSC module 400 wherein the fault current protectioncircuit has been removed. The TCSC module 400 is connected in serieswith power line 201. Module 400 comprise two branches in parallel, onebranch being the capacitor ‘C’ 304 and the second branch being theinductor ‘L’ 305 in series with the thyristor switching unit 307. Arecloser switch 306 is connected in parallel with the thyristorswitching unit 307 to shunt the unit when reclosure is necessary. Bycontrolling the firing frequency and firing angle of the thyristors inthe thyristor switching unit 307 the module is able to impress either aninductive or a capacitive impedance on the power line 201 to control thepower flow on the line 201. The control instructions and coordination ofthe TCSC 400 in distributed situations mandate coordinated action withother TCSC modules 400 and any fault protection units external to theTCSC module 400. A dedicated communication module 410 communicably linksthe TCSC module 400 to other FACTS modules, external fault protectionunits and control and coordination facility. Similar communicationmodules are used with all TCSC modules, SSSC modules and FCPM modules(fault current protection modules such as illustrated in FIGS. 6-9,though not shown therein so as to not obscure the points beingillustrated. A representative block of the TCSC module 400 is shown asblock 401.

Similar to FIG. 4, FIG. 5 shows the SSSC module 500 with the faultprotection circuitry removed. The SSSC 500 is shown as being coupled tothe power line 201 by an injection transformer, having a primary winding505 in series with the line 201 and a secondary winding 502. Similar tothe TCSC module 400, the SSSC module 500 contain the HV switches 301A to307D connected across the secondary 502 of the injection transformer aswell as a capacitor C 304 in parallel as shown. A dedicatedcommunication module 510 allow the SSSC module 500 to coordinate withother FACTS control devices, external fault protection units and controland coordination facility in a manner similar to the TCSC module 400.The SSSC 500 module is represented as a block by the equivalent block501.

FIG. 6 shows an exemplary external fault current protection module(FCPM) 600. The FCPM 600 is connected to the line 201 spanning thecircuits to be protected. It comprise the MOV 203 to handle the shortduration faults, surges and transients, a triggered gap 205 in serieswith a current limiting inductor 204 to handle longer faults, and abypass switch 206 to handle short circuits and ground short conditions.It also has a recloser switch 302 to enable the system to be reset whenthe faults are removed. The exemplary external FCPM 600 is alsorepresented by the FCPM block 601. An FCPM 601 may be hung from atransmission line or supported by a separate support, such as a separateground based or tower support, or as a further alternative, the FCPM aswell as the TCSCs and/or SSSCs may be located in a substation, such asby way of example, a substation provided specifically for that purpose.

As discussed previously, each manufacturer of the prior art FACTS devicecustom designed the FCPM to suit their design requirements andmanufacturing capabilities. By removing the non-standardized faultcurrent protection devices from the prior art TCSC 300 and the prior artSSSC, new modular and standardized TCSC 401 and SSSC 501 that handle thedesired function are made available from all FACTS manufacturers. Thesestandardized. TCSC 401 and SSSC 501 are much smaller in size, lower inweight, and usable in a distributed fashion. Having the external FCPM601 separate from the modular TCSC 401 and SSSC 501 makes arranging aplurality of these standardized FACTS modules in parallel or in serieswith a single external FCPM 601 module to handle power transferrequirements of the power grid, reducing the cost and efficiency of suchimplementation.

One of the challenges that arise when a plurality of the FACTS modulesare connected in parallel or in series, as a group, is the need forcoordinating their operation to achieve the operational goals. Highspeed and secure inter module, group to group and group to facilitycontrol is essential for the proper operation of the inter linked FACTSdevices and the single connected fault current protection module. Secureand dedicated communication techniques including line of sight wirelesscommunication using 60 and 80 Ghz bands, direct communication usinglasers etc. The challenge also extends to the operational integrationrequirement for control between the plurality of FACTS devicesconnected. This includes decision on which of the connected devicesshould be active at any point in time and when the various protectiondevices should become active.

FIG. 7 shows an exemplary block diagram 700 of implementation of theexternal FCPM 601 with FACTS modules like TCSC 401 or SSSC 501 moduleson a power grid. The block diagram 700 is similar to the FIG. 1 blockdiagram 100 and shows two sets of modular FACTS units such as TCSC 401and SSSC 501 used instead of a single unit having the fault protectionbuilt in. Each of the modular plurality of FACTS units are protected byone FCPM 601-1.

FIG. 8 shows a block diagram 800 of one arrangement of the FACTS unitssuch as SSSC 501-1 to 501-4 in a series connection with one externalFCPM 601. Two circuit breakers 103 and 106 are shown for isolating themodular FACTS units and the faulty line section between a first bus 105Aand a second bus 105B from the rest of the transmission system in caseof failure 108. Two additional ground connected breakers 801A and 801Bare also provided to allow discharging of the set of FACTS modules andthe section of the isolated transmission line when disconnected from thetransmission system using breakers 103 and 106.

FIG. 9 shows a block diagram 900 showing an alternate way for arrangingthe plurality of FACTS devices such as TCSC 401 in a parallel serialconnection. Each of the two groups of nine TCSC 401 devices 901 and 902shown as example are connected in strings of three devices and arrangedin three parallel interconnected strings. The devices are designated as401-gsp; where g is the group, s is the string and p is the position ofthe device on the string. Hence a TCSC 401 in group 2, on the secondstring at second position will be 401-222 and a TCSC 401 of the 1stgroup in the 3rd string first position will have a designation 401-131and so on. Each group of nine TCSC 401 s are shown as being protected bya single external FCPM 601.

The organization of the groups with the capability to isolate theprotected groups provide a big advantage to the serviceability of thegrid system. It is hence possible if a failure occurs in the FCPM 601module or any of the individual FACTS 401 modules, to isolate the failedmodule and replace the same with a similar module that is standardizedand pre-tested. The selective enablement of groups of FACTS 401 devicesfor power flow control and serviceability without disrupting normaloperations is hence fully enabled by the modular replacement capabilityand standardization of the FACTS 401 and FCPM 601 modules used.

The removal of the fault current protection module, by design, from eachFACTS device has numerous advantages. It reduces cost by eliminatingunnecessary duplication of heavy circuitry, itself very advantageouswhen the FACTS devices are to be hung from the transmission line. Itreduces the volume (wind forces) and the cooling requirements of eachFACTS device. It also allows and encourages standardization of the FACTSmodules in performance and control, and similarly allows independentselection of a fault current protection module design for broad use,again standardizing their performance, communication and controlrequirements. Using a fault current protection module having a recloserswitch such as switch 302 (FIG. 6), a protected group of FACTS devicescan be functionally isolated from the respective transmission line byclosing the recloser switch to remove or divert transmission linecurrent around that group of FACTS devices, which may be useful innormal operation, and particularly useful upon a failure or excessheating of a respective FACTS device in that group.

Even though the invention disclosed is described using specificimplementation, it is intended only to be exemplary and non-limiting.The practitioners of the art will be able to understand and modify thesame based on new innovations and concepts, as they are made available.The invention is intended to encompass these modifications.

Thus, the present invention has a number of aspects, which aspects maybe practiced alone or in various combinations or sub-combinations, asdesired. Also while certain preferred embodiments of the presentinvention have been disclosed and described herein for purposes ofexemplary illustration and not for purposes of limitation, it will beunderstood by those skilled in the art that various changes in form anddetail may be made therein without departing from the spirit and scopeof the invention.

What is claimed is:
 1. A method of providing distributed control andfault current protection for a power transmission and distributionsystem comprising a plurality of high-voltage (HV) transmission lines,the method comprising: providing a plurality of impedance injectionmodules distributed over and coupled to each HV transmission line, eachimpedance injection module comprising one or more interconnectedflexible alternating current transmission systems (FACTS) deviceswithout fault current protection built-in, each impedance injectionmodule configured to inject impedance into the HV transmission line;providing a plurality of external fault current protection modules, eachof the external fault current protection modules connected in parallelwith one or more impedance injection modules; and providing a pluralityof communication modules, each communication module enables an impedanceinjection module to communicate with one or more external fault currentprotection modules to handle at least a duration fault, a surge or atransient.
 2. The method of claim 1, wherein the one or moreinterconnected FACTS devices of the impedance injection module injectthe impedance into the HV transmission line, the injected impedancebeing an inductive or a capacitive impedance.
 3. The method of claim 2,wherein the one or more interconnected FACTS devices comprise aplurality of static synchronous series compensators (SSSCs).
 4. Themethod of claim 2, wherein the one or more interconnected FACTS devicescomprise a plurality of thyristor controlled series compensator (TCSCs).5. The method of claim 3, wherein one or more of the SSSCs areimplemented as part of the impedance injection module for generation andinjection of the inductive or capacitive impedance into a segment of theHV transmission line.
 6. The method of claim 4, wherein one or more ofthe TCSCs are implemented as part of the impedance injection module forgeneration and injection of the inductive or capacitive impedance into asegment of the HV transmission line.
 7. The method of claim 1 whereinthe one or more interconnected FACTS devices are coupled to the HVtransmission line using a transformer.
 8. The method of claim 1 whereinthe one or more interconnected FACTS devices of the impedance injectionmodule are connected to the HV transmission line directly without faultcurrent protection.
 9. The method of claim 1 wherein the external faultcurrent protection modules are distributed over the HV transmission lineand supported by the HV transmission line.
 10. The method of claim 1,wherein each external fault current protection module is supported by aseparate support structure.
 11. The method of claim 1, wherein the oneor more impedance injection modules and the parallel connected externalfault current protection module are located at a substation.
 12. Asystem for providing fault current protection for a power transmissionand distribution system having a plurality of high-voltage (HV)transmission lines, the system for providing fault current protectioncomprising: the plurality of interconnected flexible alternating currenttransmission systems (FACTS) devices configured as a plurality ofimpedance injection modules distributed over the HV transmission lines,the impedance injection modules being without fault current protection,each impedance injection module being coupled to an HV transmissionline; a plurality of external fault current protection modules; and aplurality of communication modules; each communication module enables animpedance injection module to communicate with an external fault currentprotection module; each external fault current protection module beingconnected in parallel across one or more impedance injection modules toprovide a coordinated fault current protection to one or moreinterconnected FACTS devices.
 13. The system of claim 12, wherein eachimpedance injection module is coupled to an HV transmission line by afirst bus having a first circuit breaker at one end and a second bushaving a second circuit breaker at another end; wherein the first andsecond circuit breakers are enabled to isolate a segment of the HVtransmission line between the first circuit breaker and the secondcircuit breaker.
 14. The system of claim 12, wherein the plurality ofinterconnected FACTS devices comprise one or more static synchronousseries compensators (SSSCs).
 15. The system of claim 12, wherein theplurality of interconnected FACTS devices comprise one or more thyristorcontrolled series compensator (TCSCs).
 16. The system of claim 12,wherein the one or more impedance injection modules and the parallelconnected external fault current protection module are distributed overan HV transmission line and are supported by the HV transmission line.17. The system of claim 12, wherein the one or more impedance injectionmodules and the parallel connected external fault current protectionmodule are distributed over an HV transmission line, with the HVtransmission line supporting the one or more impedance injection modulesand a separate support structure supporting the external fault currentprotection module.
 18. The system of claim 12, wherein each impedanceinjection module is connected to an HV transmission line using atransformer having a winding in series with the HV transmission line.19. The system of claim 12, wherein the impedance injection modules areconnected directly in series with the HV transmission lines withoutfault current protection.
 20. The system of claim 12, wherein theexternal fault current protection modules enable standardization of theimpedance injection modules in performance and power flow controlwithout fault current protection.
 21. The system of claim 12, whereinthe external fault current protection module connected in parallel withthe one or more impedance injection modules enable use of a standardizedexternal fault current protection module on the HV transmission lines.22. The system of claim 12, wherein each external fault currentprotection module comprises a plurality of fault current protectiondevices.
 23. The system of claim 12, wherein each external fault currentprotection module comprises: a recloser switch for reset, a plurality offault current protection devices selected from a group comprising ametal oxide varistor (MOV) to handle some duration faults, surges andtransients, a triggered gap in series with a current limiting inductorto handle other duration faults, and a bypass switch for short circuitconditions and ground shorts.
 24. The system of claim 12, wherein eachexternal fault current protection module comprises: a recloser switchfor reset, a metal oxide varistor (MOV) for some duration faults, surgesand transients, a triggered gap in series with a current limitinginductor for other duration faults, and a bypass switch for shortcircuits and ground faults.
 25. The system of claim 22, wherein thefault current protection devices comprise: a metal oxide varistor (MOV)for some duration faults, surges and transients, a triggered gap inseries with a current limiting inductor for other duration faults, and abypass switch for short circuits and ground faults.